1. Field of the Invention
The present invention relates to an image processing apparatus and method.
2. Related Background Art
Researches for multiplexing other information related to an image in image information have heretofore intensively been performed. In resent years, a technique called a digital watermark technique has been standardized. This technique comprises: multiplexing addition information such as an author name and permission/prohibition of use into the image information such as a photograph and picture so that the addition information is not easily visually distinguished; and distributing the information through a network such as Internet.
Moreover, as another application field, with improvement of image quality of image output apparatuses such as a copying machine and printer, for a purpose of preventing illegal counterfeiting of paper money, stamp, and securities, there has been proposed a technique of embedding the addition information into an image in order to specify an output apparatus and apparatus serial number from the image outputted onto paper.
For example, in Japanese Patent Application Laid-Open No. 7-123244, a technique has been proposed which comprises: embedding the additional information in a color-difference component low in visual sensitivity, and a high-frequency range of a chroma component to multiplex the information.
However, the above-described techniques have the following problems. FIG. 18 shows a general embedding of the additional information of the digital watermark technique. Image information A and additional information B are multiplexed via an adder 1801, and changed to multiplexed information C. FIG. 18 shows an example in which the additional information is multiplexed in a real space area of the image information. If it is possible to bring the multiplexed information C into circulation without any image processing such as various types of filtering, or any coding such as nonreversible compression, it is also easy to decode the additional information B from the multiplexed information C even in a conventional technique. For the image information which is brought into circulation on Internet and which has some resistance to noise, it is possible to decode the information even through a digital filter for image quality improvements such as edge emphasizing and smoothing.
However, it is now assumed that the multiplexed image is printed by output apparatuses such as the printer and the additional information is extracted from printed matter. Furthermore, a printer output from the printer for use which only has a representation capability of about two to several gradations per color is assumed. In recent years, an ink jet printer has been brought on the market which uses an ink having a low dye density, or variably controls a dot diameter to output, and can represent several gradations per color. However, unless a pseudo gradation process is used, gradation properties of a photographic image cannot be represented.
That is, on the above-described assumption that the multiplexed image is outputted via the printer using the digital watermark technique of FIG. 18, as shown in FIG. 19, the multiplexed information C changes to quantization information D by a pseudo gradation process unit 1901, is thereafter printed onto paper by a printer output unit 1902, and thereby changes much deteriorated information in paper F (printed matter). Therefore, when the addition information is decoded from the information in paper for the above-described purpose of counterfeit prevention, the addition information B is decoded from the information in paper E after a series of processes of FIG. 19. A change amount of the information by the processes of both units 1901, 1902 is very large, and it is very difficult to multiplex the addition information such that the information cannot visually be distinguished, and to correctly decode the multiplexed addition information on paper.
Moreover, FIG. 20 shows an example of the conventional digital watermark technique. In this example, the real space area is not used, and the image information is converted to a frequency area using Fourier transform and synthesized in a high-frequency range. In FIG. 20 the image information is converted to a frequency area by an orthogonal transformation unit 2001, and the addition information is added to a specific frequency which is not easily visually distinguished by an adder 2002. After the area is returned again to the real space area by a reverse orthogonal transformation unit 2003, similarly as the example of FIG. 18 the information is passed through the filter including large changes such as the pseudo gradation process unit and printer output unit.
FIG. 21 shows a process of separating the addition information from paper. That is, the information of the printed matter is inputted via an image reading unit 2101 such as a scanner of the printed matter. Since the inputted information is the image represented in the gradations by the pseudo gradation process unit, the image is passed through a restoration process unit 2102 as a reverse pseudo gradation process unit. In a restoration process, it is general to use a low pass filter (LPF). After the restored information is subjected to an orthogonal transformation process by a unit 2103, the embedded addition information is separated from a power having a specific frequency in a separation process unit 2104.
It is apparently seen from FIGS. 20, 21 that a large number of complicated process steps are required for multiplexing and separating the addition information. For a color image, this series of process steps also include a color change process of changing to a color peculiar to the printer. In order to realize satisfactory separation even in these complicated process steps, a signal having a very strong resistance has to be inputted. It is difficult to input the signal high in resistance while maintaining a satisfactory image quality. Moreover, with a large number of complicated process steps, a process time required for multiplexing and separating the information becomes very long.
Furthermore, in the Japanese Patent Application Laid-Open No. 7-123244, the information is added to the high-frequency area. However, when an error diffusion method is carried out in the subsequent pseudo gradation process, by characteristics of a high pass filter peculiar to the error diffusion method, a band of addition information is buried in a band of texture generated in the error diffusion, and there are possibilities of failure in decoding. Furthermore, a scanner apparatus with a very high precision is required for the decoding. That is, when the pseudo gradation process is a precondition, the method of FIGS. 19, 20 is found to be inappropriate. In other words, a multiplexing method of the addition information is required in which the characteristics of the pseudo gradation process are made heavy use of.
An example of a combination of the multiplexing of the addition information with redundancy of the pseudo gradation process is described in Japanese Patent Nos. 2640939, 2777800.
In the former proposal, for the binarizing in an organized dither method, any one is selected from dither matrices indicating the same gradation, and thereby data is mixed into the image signal.
However, in the organized dither method, unless the printer has a high resolution or very superior in mechanical precision, it is difficult to output an image having a high photographic image quality. Some deviation of the mechanical precision is generated as a low-frequency noise such as transverse streaks, and easily visually recognized on paper. Moreover, when the dither matrices are periodically changed, the band with a specific frequency is disturbed by regularly arranged dithers, and the image quality is adversely influenced.
Furthermore, gradation representation capabilities largely differ with the type of the dither matrix. Particularly on paper, a change of area ratio in an overlap of dots differs with the dither matrix. Therefore, it is also considered that a change of density is made by changing the dither matrix even in an area having a uniform density on the signal. Additionally, in a decode method of presuming the dither matrix used in the binarizing while a pixel value of the image information as an original signal is unclear on a decode (separation) side, there is a very high possibility of an incorrect decoding.
Moreover, the latter proposal relates to a method of using a dither pattern method of color to multiplex the addition information by arrangement of patterns. Even in this method, similarly as the former proposal, image quality deterioration by the changing is unavoidable. Furthermore, as compared with the former proposal, more addition information can be multiplexed, but the arrangement of color components is changed to bring about a change of hue, and the image quality is largely deteriorated particularly in a flat portion. It is also expected that the decoding on paper becomes further difficult.
In any case, both the methods of changing the dither matrix have a problem that the image quality is largely deteriorated but the decoding is difficult.
To solve the problem, the applicant of the present invention has proposed a method of using the texture generated beforehand by the error diffusion method to artificially prepare a combination of quantization values which cannot be generated in a usual pseudo gradation process, and embedding a code.
In this method, since a shape of texture only somewhat changes in a micro manner, the image quality is not visually deteriorated. Moreover, when a method of changing a quantization threshold value of the error diffusion method is used, a density value of area gradation is visually kept. Therefore, the multiplexing of various types of signals can remarkably easily be realized.
However, according to the above-described proposal, it has to be judged whether or not the texture is artificial on the decode side. In the printed matter outputted on paper, it is sometimes impossible to satisfactorily reproduce the texture by deviation from a desired shot spot position because of slippage of dots.
Moreover, in the color image, a method of multiplexing the color image in color components having a lowest visual sensitivity is a mainstream, but the distinction of the texture in the real space area is easily influenced by other color components, and it becomes difficult to separate the multiplexed information.
Furthermore, in order to solve the above-described problem, the present applicant has proposed a method of modulating amplitude of the quantization threshold value of the error diffusion method with a predetermined periodicity, controlling a plurality of types of periodicity of the threshold value modulation by an area unit, thereby controlling generation probability of the quantization value of the pseudo gradation process, and embedding the code based on the periodicity.
In the method, as compared with the above-described method of distinguishing the position and shape of the texture, relative power information in a plurality of predetermined frequency bands is an important decode factor, rather than phase information forming the code. Therefore, satisfactory decoding can be realized even on paper.
Problem to be Solved by the Invention
However, the above-described proposal has the following problem. That is, some printer for preparing the printed matter is not superior in mechanical precision.
When an ink jet printer is assumed, ink is flied and attached to a recording medium, but naturally a flied ink drop frequently deviates from the assumed shot spot position. When the slippage of the dot is generated on one line during conveyance of a recording sheet, the slippage visually becomes excessively conspicuous. Therefore, a method of reducing recoding density on the same line, and printing the information with a plurality of scans by a plurality of nozzles is general.
On the other hand, by the mechanical constitution of the printer, when the recording sheet rushes in each roller for use in conveyance, or the recording sheet comes off the roller in a step of conveying the recording sheet, the image quality is sometimes disturbed by impact vibration of the recording sheet in course of printing. As described above, since the information is printed on the same line with a plurality of divided scans in many cases, disturbance of the printing also influences a plurality of lines, and the image quality is deteriorated with a predetermined width on paper.
Moreover, there is a possibility that even left and right ends of the recording sheet are inferior in printing precision to a recording sheet center portion. That is, the image quality is not uniform on paper, and relatively satisfactory and inferior portions exist by the mechanical constitution peculiar to the printer. The portion inferior in the image quality can be regarded as a portion in which mechanical noise components are superimposed upon the image information.
When the separation of the addition information from the printed matter by the above-described proposal of the present applicant is considered, the decoding (extracting) sometimes becomes difficult in a nonuniform image quality on paper. That is, in the portion in which the image quality is relatively satisfactory, the addition information can easily be separated. However, in the portion in which the image quality is inferior, there is a possibility of failure in the separation because of the superimposed noise.
However, if the whole image is securely multiplexed in order to avoid the failure in separation, the image quality deterioration by the multiplexing becomes conspicuous. Moreover, when a decode method having a high precision is used over the whole surface of paper in order to avoid the failure of separation, a process time increases even on the decode side.